# What do you mean by transverse dimension?

Optical resonator is one of the main components of laser. Following is the extract from 'Optics' written by Ajoy Ghatak

To construct an oscillator which can supply light energy and act as a source of light, one must couple a part of the output back into the medium. This can be achieved by placing the active medium between two mirrors which reflect most of the output energy back to the system. For resonance, when a wave returns after one round trip, it must be in phase with the existing wave. For this to happen, the total phase change suffered by the wave in one complete round trip must be an integral multiple of $$2 \pi$$

Then we obtain; frequency=$$\frac{mc}{2nd}$$ . Where $$m=1,2,3...$$ . n=Refractive index of the medium enclosed by the cavity. d=Length of the cavity

In obtaining above for the various oscillating frequencies, we have assumed that a plane wave can propagate to and fro unmodified inside the resonator. This would not be true in practice since the mirrors of any practical resonator system have finite transverse dimensions and hence only that portion of the wave which strikes the mirror would get reflected; the portion of the wave lying outside the transverse dimension of the mirror will be lost from the resonator. The wave which travels back to the first mirror has now finite transverse dimensions, determined by the transverse dimensions of the mirror. Further, since only that portion of the wave that is intercepted by the mirror is reflected, the remaining portion lying outside the mirror is lost. This loss constitutes a basic loss mechanism and is referred to as diffraction loss.

Relevant portion ends here. I don't get the term transverse dimension. What is that meant? I googled it, But didn't get satisfactory definition. Please help me